Osama bin Laden's DNA: How Sure Is 99.9 Percent Sure?

Following the special forces mission that killed Osama bin Laden in Pakistan and recovered his body, reports surfaced that President Obama opted for deploying Navy SEALs, rather than a plan to bomb bin Laden's compound, because he wanted rock-solid evidence that the terrorist leader was dead. But how do you prove it's really him? We talked to genetic experts who explained how this DNA analysis was probably done, and how to understand the uncertainties inherent in any genetic test.

When President Barack Obama made his dramatic Sunday night announcement that U.S. forces had located and killed Osama bin Laden inside Pakistan, one of the first questions many asked was: How could U.S. officials be so sure it was really bin Laden?

The answer came quickly. While the body was identified through photos and by bin Laden's wife, it was also the seemingly incontrovertible proof of DNA that gave officials such ironclad confidence in the dead man's identity. "Now, we can say with 99.9 percent confidence that this was bin Laden," John Brennan, assistant to the president for homeland security and counterterrorism, said Monday. So why not complete confidence? While the body recovered from Pakistan is almost certainly bin Laden, a 100 percent identification is tougher than one might think.

It's true that DNA analysis that once required days or weeks can now be performed in less than 24 hours, particularly in high-profile cases such as this one. The process of extracting and purifying the DNA can be done in as little as a half-hour; then experts require a few hours to amplify, analyze and match it. The biggest piece of equipment needed is a genetic analyzer, a desk-size apparatus that weighs a couple hundred pounds (which could well explain why bin Laden's body had to be transported back to Afghanistan for identification).

Dan Krane, a professor of genetics and a DNA expert at Wright State University in Ohio, says modern-day rapid DNA tests can look at short tandem repeat sequences in the DNA. Tests that analyze these are indeed incredibly accurate, he says. The more closely related two people are, the more repeated patterns they will share.

But Krane and others—such as William Thompson, a criminology professor at the University of California, Irvine, and an expert in DNA forensics—say that the 99.9 percent confidence number, which the U.S. government reportedly arrived at by comparing the DNA to that of bin Laden's half-sister, is slightly misleading. The key distinction, Thompson says, is that the results speak to the probability of a relationship between the two people, not the actual identity of the dead man. "If the DNA test compared the profile of the man shot in Pakistan with bin Laden's family members," Thompson says, "the results could properly be presented only as a likelihood ratio, stating the relative likelihood of observing the particular markers found in the dead man if he were, say, the father or brother of a known bin Laden family member, than if he were a randomly chosen individual."

Kane notes that the more appropriate (though perhaps less dramatic) way to explain the results would be as such: It is about 1000 times more likely the dead man's DNA came from a sibling of bin Laden's sister than from a random person. "That phrasing is more consistent with how paternity and sibling tests are typically described," he tells PM.

One complicating factor in identifying bin Laden is that he has no full siblings. "In the case of half-siblings, it is harder to state that there is a match because all one can do is state that it is likely the two people shared a common ancestor (you would not be able to say they are half-brother and -sister any more than you could say they are cousins)," Steven Laken, the CEO of DNA analysis firm Cephos, writes in an email to PM. "In the case of the bin Ladens, there may have been consanguineous marriages (marriages between related people), and this makes it trickier."

Another factor is how and when bin Laden family DNA was collected. It's been widely reported that the federal government collected DNA from bin Laden's half-sister when she died in a Massachusetts hospital, and that DNA from other bin Laden relatives may also have been accrued and used in the identification (though the government isn't providing all the details at this time). It's no secret that the Pentagon samples the DNA of suspected and known terrorists and their family members. A 2007 Defense Science Board report revealed Black Helix, an initiative­­—apparently dating back to early 2001, before the 9/11 attacks—that provides a "secure repository and interactive database, which will focus on archiving, retrieving, and interpreting biomolecular data for the identification and tracking of terrorist suspects."

Do such databases—combined with modern scientific techniques—make high-profile identifications a done deal? Not always, says Steven Aftergood of the Federation of American Scientists. He cautions that no matter how careful or thorough one is, genetic testing always presents the possibility of errors, ranging from contamination to simple lab mixups. "I'm not suggesting that was the case with the bin Laden DNA," he says. "But life is complicated."